Publish an Initial Roadmap (#2804) (#2810)

* Add initial roadmap
Co-authored-by: edwardcwang <edwardcwang@users.noreply.github.com>

(cherry picked from commit 7f73d18ecd2b1cf63485f3c268456ba0c775b961)

Co-authored-by: Megan Wachs <megan@sifive.com>

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+# Chisel Roadmap
+
+This roadmap captures the current plans of the Chisel maintainers and can be
+changed through the usual PR process.
+Not every small feature add or planned bugfix is logged here, but the high-level
+changes and planned direction that are relevant for those using and contributing
+to the project.
+Two especially notable directions on the roadmap below are:
+
+* We are migrating Chisel to rely on, exclusively, the
+  [MLIR-based FIRRTL Compiler](https://github.com/llvm/circt)
+* Accelerating improvements and changes to Chisel and the FIRRTL specification
+
+
+## 3.5.x (and Earlier)
+
+- Continue to maintain binary compatibility on these releases
+- Continue to backport bug fixes, new features if possible, documentation
+  updates, etc.
+- Deprecations are backported to ease in users migrating to next major release
+  (3.6).
+
+## 3.6 (Target 2022 November)
+
+This release is intended to provide a smooth path for the Chisel codebase to no
+longer depend on the 
+[Scala FIRRTL Compiler (SFC)](https://github.com/chipsalliance/firrtl)
+and use the [MLIR Firrtl Compiler (MFC)](https://github.com/llvm/circt) as the
+main backend. While the SFC may continue to work to consume the `.fir` produced
+by Chisel, it will not be a requirement for adding new features to the FIRRTL
+spec and the Chisel APIs. Thus, the 3.6 release will:
+
+- Deprecate `Chisel._`
+- Turn the deprecations from previous releases into Errors
+- Support integration with [MFC](https://github.com/llvm/circt) by bringing
+  `chisel-circt` in-repo
+- Stop using [SFC](https://github.com/chipsalliance/firrtl) except to test
+  deprecated features
+- Deprecate all Chisel features that are not implemented in MFC. A nonexhaustive
+  list includes:
+  - Interval Types
+  - Fixed Point Types
+  - Injecting Aspects
+  - EnumAnnotations
+  - RunFirrtlTransformAnnotation
+  - LoadMemoryFromFile
+  - ChiselAnnotation/custom annotations
+- Publish Roadmap
+
+
+## 4.0
+
+See the Q&A below.
+
+## 5.0.0 (Target Q1 2023)
+
+As we look forward to Chisel development, we plan to accelerate coordinated
+changes with the
+[FIRRTL specification](https://github.com/chipsalliance/firrtl-spec) to add new
+functionality. The bump to version 5.0.0 will be a major change in the overall
+versioning scheme of the Chisel project. We will switch to
+[Semantic Versioning 2.0.0](https://semver.org/), which means we expect the
+MAJOR version number to increment much more frequently. To make this change more
+apparent, these changes will be made in the Chisel 5.0.0 release:
+
+- The build artifact will be renamed to `org.chipsalliance.chisel`
+- The repository will be renamed to github.com/chipsalliance/chisel (the
+  `chisel3` URL will still work as an alias).
+- All of Chisel’s internal tests will rely on MFC, not SFC, for lowering from
+  firrtl
+- Delete Chisel._
+- The repo will be flattened with its dependencies/dependents as much as
+  possible
+  - Bring all things in from SFC into chisel repo that are needed(FIRRTL IR,
+    Stage, etc)
+  - chisel-test (or a subset of it that works with MFC) will be brought into the
+    chisel repo
+  - The chisel-lang website will be brought in-repo
+- Only the chisel repo will be released as an artifact (as all supported
+  dependents and dependencies will be flattened in)
+
+## Beyond 5.0.0
+
+Faster co-improvements to Chisel, MFC, the FIRRTL IR and Chisel.
+
+## Q & A
+
+* What happened to Chisel 4?
+
+We want to make it clear that there is a change in development versioning and
+philosophy (moving faster), so we are skipping 4 and moving straight from
+chisel3 to Chisel 5.0.0.